(Invited Speaker) Marine stratcumulus cloud decks—Natural laboratories for inadvertent and planned cloud seeding experiments

Marine stratocumulus cloud decks off the west coast of continents provide an ideal environment for using cloud seeding—inadvertent and intentional--to address basic science questions concerning aerosol-cloud-precipitation interactions. Marine stratocumulus systems are often associated with relatively slowly evolving synoptic and surface conditions. Thus point, line, or area sources of aerosols that are introduced into these cloud systems can produce cloud signatures that can be compared with steady conditions in the surrounding clouds. In some cases, causes and effect experiments are possible. Cloud modifications of two types have been considered. The first is a brightening of clouds associated with increases in CCN as cloud water is distributed among smaller droplets than those in the environment and possible increases in liquid water path (LWP) as increased aerosols reduce precipitation efficiency relative to that in the environment. The possibility that enhanced evaporation of cloud droplets near cloud top may also play a role in the modification process has also been considered. The second type of modification involves increasing precipitation efficiency by introducing giant CCN into clouds that are susceptible for modification. One complication that affects the interpretation of stratocumulus seeding experiments is that mesoscale structure in the form of closed and open cells can complicate the resulting response of the clouds to aerosol perturbations. Further, natural cloud-aerosol-interaction process may mask the variability associated with seeding sources.

In this presentation an assessment of our current knowledge and understanding of marine stratocumulus modifications associated with ship emissions (ship tracks) and coastal sources of aerosols will be made. Recent attempts at intentionally seeding marine stratocumulus clouds—true cause and effect experiments-- will be discussed and followed by an evaluation of proposals for seeding marine stratocumulus clouds for science process studies and for possible modification of the Earth's climate.